An outline of a conventional passive containment cooling system of a nuclear power plant will be described with reference to FIGS. 11 to 14.
FIG. 11 is a sectional elevational view showing an example of a configuration of the conventional passive containment cooling system. In FIG. 11, a core 1 is contained in a reactor pressure vessel 2. The reactor pressure vessel 2 is contained in a containment vessel 3. The containment vessel 3 has a cylindrical shape (see FIG. 12).
The interior space in the containment vessel 3 is partitioned into a dry well 4, which contains the reactor pressure vessel 2, and a wet well 5. The dry well 4 and the wet well 5 each constitutes a part of the containment vessel 3. The wet well 5 forms a suppression pool 6 inside. A wet well gas phase 7 is formed above the suppression pool 6. The outer wall parts of the dry well 4 and the wet well 5 are integrated to constitute a cylindrical outer wall part of the containment vessel 3. The ceiling part of the dry well 4 is a flat plate, which will be referred to as a top slab 4a of the dry well 4.
In the case of a boiling water reactor, the atmosphere in the containment vessel 3 is inerted by nitrogen and limited to a low oxygen concentration. In the case of the boiling water reactor, the containment vessel 3 is contained in a nuclear reactor building 100.
In general, there are various types of containment vessels 3 depending on the materials. Examples include a steel containment vessel, a reinforced concrete containment vessel (RCCV), a pre-stressed concrete containment vessel (PCCV), and a steel concrete composite (SC composite) containment vessel (SCCV). In the cases of RCCV and PCCV, the inner surfaces are lined with a steel liner. FIG. 11 shows an example of an RCCV. As shown in FIG. 12, an RCCV has an outer wall part of cylindrical shape.
The reactor pressure vessel 2 is supported by a cylindrical pedestal 61 via an RPV skirt 62 and an RPV support 63. The pedestal 61 may be made of steel, concrete, or a composite structure of both. In the dry well 4, the inside space of the pedestal 61, below the reactor pressure vessel 2 and surrounded by the cylindrical wall of the pedestal 61, is referred to as a pedestal cavity 61a. In the case of the RCCV of an ABWR, the cylindrical wall of the pedestal 61 forms a boundary wall between the wet well 5 and the dry well 4. The space is referred to as a lower dry well in particular.
A containment vessel head 10 is arranged above the reactor pressure vessel 2. A water shield 11 is arranged over the containment vessel head 10.
Main steam pipes 71 extend from the reactor pressure vessel 2 to outside the dry well 4. A safety relief valve (SRV) 72 is arranged on the main steam pipes 71. A discharge pipe 73 is arranged to be submerged in the suppression pool 6 so that the steam in the reactor pressure vessel 2 is released into the suppression pool 6 if the safety relief valve 72 is activated.
The dry well 4 and the suppression pool 6 are connected by LOCA vent pipes 8. There are installed a plurality of, for example, ten LOCA vent pipes 8, whereas FIG. 11 shows only two of them (see FIG. 12). The LOCA vent pipes 8 have horizontal vent pipes 8a in the portions submerged in the pool water of the suppression pool 6. The horizontal vent pipes 8a open in the pool water. In the case of an RCCV, three horizontal vent pipes 8a are vertically arranged on each LOCA vent pipe 8. In the case of the RCCV the LOCA vent pipes 8 are installed through the cylindrical wall of the pedestal 61. In the case of the RCCV, the cylindrical wall of the pedestal 61 is thus also referred to as a vent wall. The vent wall is made of reinforced concrete with a thickness of approximately 1.7 m. The inner and outer surfaces are made of steel. The LOCA vent pipes 8 and the pedestal 61 constitute a part of the containment vessel 3.
Vacuum breakers 9 are provided for the purpose of letting the gas in the wet well gas phase 7 flow back into the dry well 4. There are provided a plurality of, for example, eight vacuum breakers 9, whereas FIG. 11 shows only one of them.
The vacuum breakers 9 may be formed on the wall surface of the wet well 5, on the ceiling of the wet well 5, and on the LOCA vent pipes 8. The vacuum breakers 9 are activated to open if the pressure in the wet well 5 exceeds that in the dry well 4 and the difference in pressure exceeds a set pressure difference. For example, the set pressure difference of the vacuum breakers 9 is approximately 2 psi (approximately 13.79 kPa). The vacuum breakers 9 constitute a part of the containment vessel 3.
A cooling water pool 13 of a passive containment cooling system 12 is arranged outside the containment vessel 3. The cooling water pool 13 stores cooling water 14 inside. FIG. 11 shows an example of the cooling water pool 13 of a tank type, whereas the cooling water pool 13 may be of a pool type. In the case of the pool type, the cooling water pool 13 is covered with a lid from above. FIG. 11 shows an example where the cooling water pool 13 and the like are installed inside the nuclear reactor building 100. The cooling water pool 13 and the like may be installed in an adjacent auxiliary building or the like.
An exhaust port 15 for releasing steam to the environment is extended from the gas phase above the water surface in the cooling water pool 13. An insect screen may be arranged on the outlet of the exhaust port 15. The cooling water pool 13 is usually located above the containment vessel 3. The cooling water pool 13 may be arranged beside the containment vessel 3.
A heat exchanger 16 is installed in the cooling water pool 13 to be submerged at least in part in the cooling water 14.
A plurality of the heat exchangers 16 may often be installed, although FIG. 11 shows only one heat exchanger 16. The heat exchanger 16 includes an inlet plenum 17, an outlet plenum 18, and heat exchanger tubes 19 (see FIG. 13).
FIG. 11 shows an example in which only the heat exchanger tubes 19 are installed inside the cooling water pool 13, and the inlet plenum 17 and the outlet plenum 18 (FIG. 13) protrude out of the cooling water pool 13. However, the configuration is not limited to this example. For example, the entire heat exchanger 16, including the inlet plenum 17 and the outlet plenum 18, may be installed inside the cooling water pool 13.
The inlet plenum 17 is connected with a gas supply pipe 20 for supplying the gas in the dry well 4. One end of the gas supply pipe 20 is connected to the dry well 4.
The outlet plenum 18 is connected with a condensate return pipe 21 and a gas vent pipe 22. One end of the condensate return pipe 21 is connected to inside the containment vessel 3. FIG. 11 shows an example in which the condensate return pipe 21 is led into a LOCA vent pipe 8. However, the configuration is not limited to this example. For example, the condensate return pipe 21 may be led into the dry well 4 or into the suppression pool 6.
The installation into the LOCA vent pipe 8 has a problem of increasing the pressure loss of the LOCA vent pipe 8 at the time of LOCA. The installation into the dry well 4 needs a PCCS drain tank installed in the dry well 4 for the sake of water sealing, and is thus not able to be employed unless there is room in the dry well 4. The installation into the suppression pool 6 increases the length of the condensate return pipe 21 outside the PCV, and thus has a problem that the possibility of leakage of radioactive materials increases.
One end of the gas vent pipe 22 is led into the wet well 5 and installed to be submerged in the suppression pool 6. The gas vent pipe 22 is installed so that the submerging depth in the suppression pool 6 is smaller than the submerging depth of the topmost ends of the openings of the LOCA vent pipes 8 in the suppression pool 6.
FIG. 13 is a sectional elevational view showing an example of the heat exchanger of the conventional passive containment cooling system. Referring to FIG. 13, the structure of the heat exchanger 16 of the conventional passive containment cooling system 12 will be described by using a horizontal heat exchanger as an example.
In FIG. 13, the outlet plenum 18 is arranged below the inlet plenum 17. A large number of U-shaped heat exchanger tubes 19 are connected to a tube plate 23. The straight portions of the heat exchanger tubes 19 are installed horizontally. FIG. 13 shows only two of the heat exchanger tubes 19 in a simplified manner. The outside of the heat exchanger tubes 19 is filled with the cooling water 14 (see FIG. 11). The inlets of the heat exchanger tubes 19 are opened to the inlet plenum 17. The outlets of the heat exchanger tubes 19 are opened to the outlet plenum 18.
The gas supply pipe 20 is connected to the inlet plenum 17, and supplies a mixed gas of nitrogen, oxygen, steam and the like in the dry well 4 to the inlet plenum 17. The mixed gas is led into the heat exchanger tubes 19. The steam condenses into condensate, which flows out of the outlets of the heat exchanger tubes 19 into the outlet plenum 18 and accumulates in the lower part of the outlet plenum 18.
The condensate return pipe 21 is connected to the lower part of the outlet plenum 18. The condensate return pipe 21 returns the condensate in the outlet plenum 18 into the containment vessel 3 by gravity. The gas vent pipe 22 is connected to the upper part of the outlet plenum 18. Noncondensable gases that do not condense in the heat exchanger tubes 19, such as nitrogen and hydrogen, are discharged from the heat exchanger tubes 19 and accumulate in the upper part of the outlet plenum 18.
The end of the gas vent pipe 22 is led to the suppression pool 6. The noncondensable gases in the outlet plenum 18 pass through the gas vent pipe 22, push down the pool water in the suppression pool 6, are vented into the pool water and then move to the wet well gas phase 7.
Note that the shape of the heat exchanger tubes 19 is not limited to the U shape. There is a structure installing upright heat exchanger tubes 19 with vertical straight tube portions. The inlet plenum 17 is always located above the outlet plenum 18. In such a manner, the condensate condensed in the heat exchanger tubes 19 is led into the outlet plenum 18 by gravity. The advantages of the horizontal type are good seismic performance and effective utilization of the cooling water 14. On the contrary the advantage of the vertical type is high drainability of the condensate.
Next will be described functions of the conventional passive containment cooling system 12 configured as such.
If a loss-of-coolant accident (LOCA) with a break of a piping in the dry well 4 occurs, steam is generated in the reactor pressure vessel 2, which sharply increases the pressure in the dry well 4. The gas (mostly nitrogen and steam) in the dry well 4 passes through the gas supply pipe 20 of the passive containment cooling system 12 and is supplied to the heat exchanger 16.
The noncondensable gases having accumulated in the outlet plenum 18 of the heat exchanger 16 passes through the gas vent pipe 22 and is discharged into the suppression pool 6. The discharge of the noncondensable gases is let by a pressure difference between the dry well 4 and the wet well 5.
At the time of the LOCA, the pressure in the dry well 4 is higher than that in the wet well 5. The discharge of the noncondensable gases is thus performed smoothly. Consequently, the gas in the dry well 4 becomes mostly steam in some time. In such a state, the heat exchanger 16 can efficiently condense the steam in the dry well 4 and return (or circulate) the condensate into the containment vessel 3.
Immediately after the occurrence of the LOCA, a large amount of steam generates from the coolant, and rapid venting of the gas in the dry well 4 to the wet well 5 is mostly performed through the LOCA vent pipes 8.
The steam condenses in the suppression pool 6 while noncondensable nitrogen does not condense in the suppression pool 6 and moves to the wet well gas phase 7. By the rapid venting through the LOCA vent pipes 8, most of the nitrogen in the dry well 4 moves to the wet well 5, for example, in about one minute after the LOCA.
Subsequently, the vent flowrate decreases. Since the submerging depth of the gas vent pipe 22 in the suppression pool 6 is set to be smaller than that of the LOCA vent pipes 8, the gas in the thy well 4 starts to be vented to the wet well 5 through the gas vent pipe 22 in some time after the LOCA.
In such a manner, the vent flowrate subsides and the steam generated in accordance with the decay heat of the core fuel is released from the LOCA break into the dry well 4. It is designed that the steam is led through the gas supply pipe 20 into the heat exchanger 16 for cooling, but not through the LOCA vent pipes.
As a result, because the decay heat of the core fuel is transferred to the outside cooling water 14, the increase of pressure in the containment vessel 3 due to heat up of the water in the suppression pool 6 can be prevented. The passive containment cooling system 12 is thus designed to be able to passively cool the containment vessel 3 without using external power at all.
Next, in the case of a transient event such as a station blackout (hereinafter, may be referred to as “SBO”), the decay heat generated in the core is transferred to the suppression pool 6 by the reactor steam passing through the safety relief valve 72. As the reactor steam condenses in the suppression pool 6, the decay heat is transferred to the pool water and the temperature of the pool water increases gradually. After a lapse of certain time, the pool water is saturated and steam equivalent to the decay heat flows continuously into the wet well, gas phase 7 to pressurize the wet well gas phase 7. This activates the vacuum breakers 9, and the nitrogen and steam in the wet well gas phase 7 flow into the dry well 4. The dry well 4 is thereby pressurized, and the nitrogen and steam in the dry well 4 are led to the heat exchanger 16 of the passive containment cooling system 12 through the gas supply pipe 20, whereby the steam is condensed.
Since the nitrogen, which is a noncondensable gas, simply remains in the heat exchanger 16, the passive containment cooling system 12 stops functioning. The reason is that although the gas vent pipe 22 is led from the heat exchanger 16 to the suppression pool 6, the pressure of the wet well gas phase 7 increases under the SW and so the noncondensable gas in the heat exchanger 16 is not able to be vented to the wet well gas phase region 7.
To solve such a problem, Patent Document 1 discloses a method of providing an outer well 32 outside the dry well 4 and the wet well 5, and leading the gas vent pipe 22 into a water seal pool retained therein to release the noncondensable gas accumulated in the heat exchanger 16 into the outer well 32 (see FIG. 2 of Patent Document 1). The interior of the outer well 32 is inerted by nitrogen in consideration of the prevention of detonation even when hydrogen is vented.
Patent Document 2 discloses a method of connecting the gas supply pipe 20 to the wet well gas phase 7 to directly lead the steam and nitrogen in the wet well gas phase 7 to the heat exchanger 16, and discharging noncondensable gases such as nitrogen accumulated in the heat exchanger into the dry well 4 by using an exhaust fan 24 arranged on the gas vent pipe 22 (see FIG. 2 of Patent Document 2). In either case, the gas supply pipe 20, the condensate return pipe 21, and the gas vent pipe 22 are installed outside the containment vessel 3.
In preparation for a core meltdown in the event of a transient event such as a station blackout (SBO), ABWRs to be built in Europe and the U.S.A. have fusible valves 64 and lower dry well flooder pipes 65 inside the pedestal cavity 61a. The lower dry well flooder pipes 65 are extended from the LOCA vent pipes 8 through the wall of the pedestal 61 and connected to the fusible valves 64. The fusible valves 64 and the lower dry well flooder pipes 65 are installed on all the LOCA vent pipes 8. If the temperature of the lower dry well 61a reaches approximately 260 degrees Celsius, low melting point plug portions of the fusible valves 64 melt to open. At the time of a core meltdown accident, the corium melts the bottom of the reactor pressure vessel 2 through and falls into the pedestal cavity 61a. This increases the temperature in the pedestal cavity 61a abruptly, and the fusible valves 64 open and the cooling water in the LOCA vent pipes 8 flows into the pedestal cavity 61a through the lower dry well. flooder pipes 65 to flood and cool the corium.
Other examples of the valves for pouring water on the fallen high-temperature corium with the same purpose as that of the fusible valves 64 include squib valves and spring valves. ESBWRs (Economic Simplified Boiling Water Reactors) use squib valves. EPRs (European Pressurized Reactors) use spring valves. A large amount of steam generated at that time flows into an upper dry well from openings 66 in the LOCA vent pipes 8, passes through the gas supply pipe 20, and is led to the heat exchanger 16 of the passive containment cooling system 12 for condensation. Meanwhile, the noncondensable gases accumulated in the heat exchanger 16 are vented into the wet well 5 through the gas vent pipe 22. In such a state, the pressure in the dry well 4 is higher than that in the wet well 5, so that the noncondensable gases are efficiently vented to the wet well gas phase 7. The condensate returns to a LOCA vent pipe 8 through the condensate return pipe 21, passes through the lower dry well flooder pipe 65, and is used to cool the corium again.
In addition, the pool water in the LOCA vent pipes 8 is also supplied from the suppression pool 6 through the horizontal vent pipes 8a. 
The fusible valves 64 and the lower dry well flooder pipes 65 described above have had a problem; that is, if the pressure in the dry well 4 increases after the fusible valves 64 are opened, the high-temperature water pooled in the lower dry well 61a flows back into the suppression pool 6 to increase the temperature of the suppression pool water. Backflow prevention measures have been difficult to be implemented because the temperature of the portions of the lower dry well flooder pipes 65 in the lower dry well 61a become so high at the time of an accident that it is difficult to expect devices to function. The installation of devices inside the LOCA vent pipes 8 is also difficult since they interfere with the safety function of the vent pipes. Hence, the prevention measures were difficult.
FIG. 4 of Patent Document 2 discloses a method for leading the condensate condensed in the heat exchanger 16 to a PCCS drain tank 76 by the condensate return pipe 21. It is further disclosed to provide an overflow pipe 77 on the gas phase region of the PCCS drain tank 76 to return overflow water into the containment vessel 3. However, the condensate return pipe 21, the PCCS drain tank 6, and the overflow pipe 77 are all installed outside the containment, vessel 3, and radioactive materials may possibly leak from such devices to the outside environment.
Patent Document 3 discloses a method for providing a PCCS drain tank in the dry well and injecting cooling water in the PCCS drain tank into the containment vessel by gravity by using an injection pipe. However, according to such a method, the PCCS drain tank is installed in the dry well. In the case of the RCCV used for the ABWR, there no room to spare and the method has been impossible to be implemented.
Next, a conventional filtered venting system will be described with reference to FIG. 14. A filtered venting system 50 has been employed in European nuclear power plants after the accident at the Chernobyl nuclear power plant.
FIG. 14 is a sectional elevational view showing a design example of the conventional filtered venting system. The filtered venting system 50 includes a filtered venting tank 51 storing decontamination water 52, an inlet pipe 53 for leading gas in the containment vessel 3 to the decontamination water 3, and an outlet pipe 54 for releasing gas in the gas phase region of the filtered venting tank 51 to the environment. The upper portion of the outlet pipe 54 passes through a stack 75.
The installation location of the filtered venting tank 51 and the like is not limited to inside the building. If the filtered venting tank 51 and the like are installed in an existing reactor as a backlit, the filtered venting tank 51 and the like are often installed outside the nuclear reactor building. If installed from the beginning of construction, the filtered venting tank 51 and the like may be installed inside the nuclear reactor building and the like.
A Venturi scrubber 55 may be installed in the decontamination water 52 so that the gas led from the inlet pipe 53 passes through the Venturi scrubber 55. However, the Venturi scrubber 55 is not indispensable. A metal fiber filter 56 may be installed in the gas phase region of the filtered venting tank 51, although the metal fiber filter 56 is not indispensable.
FIG. 14 shows a case in which both the Venturi scrubber 55 and the metal fiber filter 56 are installed. For example, one isolation valve 57 is installed on the inlet pipe 53. A rupture disc 58 is arranged in parallel with the isolation valve 57, and normally-opened isolation valves 59a and 59b are arranged in front of and behind the rupture disc 58. Two isolation valves 57 may be connected in series.
An outlet valve 60 is installed, though not indispensable, on the outlet pipe 54. A rupture disc is often used instead of a motor-driven valve, in the conventional filtered venting system, one end of the inlet pipe 53 is directly connected to the containment vessel 3 to take in the gas inside the containment vessel 3. The filtered venting system can efficiently remove particulate radioactive materials, such as CsI, with a DF (Decontamination Factor) of approximately 1,000 to 10,000. However, because the conventional filtered venting system cannot remove radioactive noble gases or organic iodine, those radioactive materials are released to the environment through the outlet pipe 54 when it is activated.
The filtered venting tank 51 of the conventional filtered venting system has a limited size, often with a decontamination water (scrubbing water) capacity of no more than 100 m3. If radioactive materials are removed, the decontamination water 52 thus evaporates and decreases due to the heat generated by the radioactive materials. In the event of an actual severe accident, the decontamination water has therefore needed to be replenished from outside.
For powder separators, a cyclone separator by M. O. Morse (1886) has been widely used in sawmills, oil refining facilities, etc. A cyclone separator is an application of the principle of a centrifugal separator. A solid-containing liquid or gas is made to flow circumferentially into a funnel-like or cylindrical cyclone to trace a spiral by the flow of the gas or liquid. The gas or liquid is discharged upward from the center of the circle of the cyclone. The solid is centrifugally separated, collides with the wall surface, then falls by gravity, and accumulates below. With such a mechanism, the gas or liquid are discharged from the center of the circle after most of the solid components are removed. To collect the separated solid components, a collection container is often arranged under the cyclone. As the speed of the fluid flowing in from the inlet increases, the centrifugal force increases and the removal, efficiency of the cyclone separator improves.